Understanding a Marine Exhaust System

Website design By BotEap.comMarine exhaust systems are an important and often overlooked part of a marine engine. Most boats have a “wet” exhaust system. The salt water is injected into the riser, the exhaust outlet of the exhaust manifold. This mixture of exhaust gases and water is then passed through a series of curves until it exits the boat, preferably at the stern.

Website design By BotEap.comExhaust gases mixed with salt water create a highly corrosive compound. This is why exhaust systems are commonly made with non-corrosive components, such as hoses reinforced with nitrile rubber (brown stripes, not green), galvanized steel, fiberglass, or plastic. Varying in size and shape depending on engine size and engine room layout, the purpose of these components is to bypass engine hydraulics. This occurs when an engine has filled with salt water that has entered the exhaust and can cause extensive damage if left for more than two hours. Water enters the exhaust from wave action at the stern and poor exhaust design. In some ocean conditions, such as swells, water can back up the exhaust when the engine is not running. Poorly designed exhausts allow water to flow back and fill the waterlock/muffler box and then up the hose into the exhaust manifold, through the exhaust valves and into the combustion chamber. With the engine filled with water, the engine cannot start because it cannot compress water.

Website design By BotEap.comYou’ll know your engine is hydraulic if it won’t turn over, usually after a long period of boating, and you’ve removed the starter and made sure it’s running. To remove water from inside the combustion chambers, first remove all injectors, crank engine to expel water, replace injectors, bleed injector lines, and start. Then let the engine run until the leak is fixed.

Website design By BotEap.comCheck that the riser is not coked or corroded, a common problem. To check the exhaust riser, remove the exhaust hose from the riser (often a difficult procedure) and look up to see if it is clogged by salt buildup or exhaust. If the buildup is excessive, it will be necessary to remove the riser to check the tube end of the motor. The coke can be scraped off to provide a short-term fix, although the riser will often need to be replaced. There are aftermarket systems that vary in quality. Be sure to set the correct layout for the application.

Website design By BotEap.comMake sure your waterlock is low and large enough to contain all the water in the exhaust system. Is there a gooseneck or central vertical loop in the exhaust hose at the transom outlet? Is there a siphon break and is it working properly? No leaky valves?

Website design By BotEap.comExhaust fumes are poisonous and can cause dizziness and headaches. Replace any defective part immediately. Use double hose clamps at each joint or preferably super clamps, bolt style and cement exhaust if necessary. Hot sections should be covered with fiberglass tape to prevent burns.

Website design By BotEap.comBeware of asbestos siding. Many older vessels and marine engine installations had exhaust systems lined with asbestos tape and rope. Asbestos soundproofing was also common on older ships.